The present invention relates to an improvement of a caster for a wheelchair or the like, particularly a caster having a shock absorbing mechanism.
Heretofore, there has been a caster having a shock absorbing mechanism which a link to link a wheel to a body of a wheelchair is connected to the body through a rubber bush and the rubber bush is twisted due to displacement of the wheel resulting from a variation in load, thereby absorbing a shock, or the link is replaced with a hydraulic damper having both the functions of a link and a hydraulic damper.
However, in the case of a rubber bush type, usually a twist of the rubber bush is not enough for the damping of a shock after applied. On the other hand, in the case of a hydraulic damper type, its spring characteristic is constant, so that the hydraulic damper is expanded and contracted due to a load applied. This causes a great variation in position of the wheel or occurrence of a resonance phenomenon depending on how uneven the surface of a road is.
The present invention is made in consideration of the problems described above. An object of the present invention is to provide a caster having a shock absorbing mechanism which can cope with a variation in natural frequency of the body of a wheelchair resulting from a variation in load or can stabilize a posture of the body of a wheelchair.
In order to solve the problems mentioned above, the present invention is made and characterized in that in a caster which supports a wheel via a link by using a bracket to be fitted to a wheelchair or the like, an end of the link is coupled to a rotation axis of the bracket through a rubber member which is twisted and deformed resulting from oscillating motion of the link and also the link and the bracket are connected by a hydraulic damper which damps the oscillating motion of the link, whereby the rubber member to connect the wheel and the bracket is twisted and deformed according to a load of the wheelchair, an inclination of the link is determined, and the oscillating motion of the link resulting from vertical movements of the wheel is damped by the hydraulic damper.
As described above, due to a twisting spring characteristic of the rubber bush and a shock absorbing characteristic of the hydraulic damper, it is possible to absorb and promptly damp a shock applied resulting from an oscillation or a change of the surface of a road while traveling.
Since the aforementioned link is composed of a variable link the length of which varies, even though a load applied to the wheelchair varies, it is always possible to cope with a variation in natural frequency of the wheelchair by changing the length of the link. Therefore, the oscillation of the wheelchair can be suppressed or the posture of the wheelchair can always be maintained constant.
Further, the aforementioned variable link is composed of a support cylinder, a movable shaft which is slidably inserted in the support cylinder, positioning holes provided at the support cylinder and the movable shaft, and positioning pins for adjusting a longitudinal position of the movable shaft.
There are two levers which are connected between the bracket and the link so as to fix the wheel at a predetermined position and one of the levers is equipped, at a side end of a section of connecting both of the levers, with a restraint mechanism having a claw for locking relative positions of these two levers. Therefore, an oscillation of the wheelchair can be suppressed, for example, when a heart massage is given to a sick person on a wheelchair, it can efficiently be carried out.
It is arranged such that a viscous hydraulic operating fluid, for example, silicon oil is filled as a hydraulic operating fluid for the hydraulic damper and a pressure of the silicon oil supports a load to be applied to the bracket. Therefore, the hydraulic damper compresses and also shrinks the silicon oil according to the load applied to the body of the wheelchair, whereby an inclination of the link is determined. If a heavy load is applied to the body, a pressure will make a steep rise due to the compression characteristic of the silicon oil resulting from the shrinkage of the hydraulic damper, whereby a depression of the caster can be suppressed and movements of the link can be secured. If a compression ratio of the silicon oil is changed, a pressure buildup curve of the hydraulic damper will vary and damping characteristics of every kind can be obtained.
Since the silicon oil has a compressibility higher than that of a hydraulic operating fluid used for general hydraulic equipment, it is not necessary to install a gas chamber or the like in the hydraulic damper, whereby the structure can be simplified. Further, the silicon oil has elasticity, and therefore it has a function of supporting a load to be applied to the body of the wheelchair, whereby it is possible to eliminate the rubber bush.
The aforementioned hydraulic damper is composed of a damper tube which is connected with one of the bracket and the link described above, a piston rod which is connected with the other of the bracket and the link, a piston which is connected with the piston rod and partitions the inside of the damper tube into two oil rooms, and a space which is formed between the damper tube and the piston and connects the aforementioned two oil rooms. Therefore, the hydraulic damper is expanded and contracted according to a shock applied from the surface of a road to the wheel and viscous resistance of the silicon oil which passes the space makes a steep rise according to the operating speed, whereby an oscillation of the link is effectively damped.
Since the hydraulic damper fulfills both a spring function of supporting a load applied to the body of the wheelchair and a damper function of damping an oscillation of the link, the number of parts of the caster can be curtailed and thus the caster can be miniaturized.
Since a cylindrical section to be engaged with the piston rod and a discoid section in which the aforementioned space is formed to divide the discoid section from the damper tube are formed at the piston, a size of the piston can easily be changed without modifying a design of the piston rod or the like. By changing the size of the piston and thus changing the cubic volume of the silicon oil to be filled in the hydraulic damper, it is possible to change the damping characteristic of the hydraulic damper.